1. Field of the Invention
The present invention relates generally to constant voltage circuits, constant current sources, amplifiers, and power supply circuits, and more particularly to a constant voltage circuit capable of reducing the charge time of a capacitor for noise prevention, and a constant current source, an amplifier, and a power supply circuit using the constant voltage circuit.
2. Description of the Related Art
In recent years, there has been a demand for power saving also in terms of environmental measures. The importance of reduction in power consumption in apparatuses using batteries, such as cellular phones and digital cameras, is increasing also in terms of extending the useful service life of the batteries. As a result, in those apparatuses, the bias current of an amplifier circuit and the current consumed in a constant voltage circuit such as a reference voltage generator circuit are extremely reduced. Consequently, the circuits themselves have become more susceptible to noise, so that a capacitor for noise absorption is provided in the circuits.
FIG. 1 is a schematic circuit diagram showing a conventional constant voltage circuit 100 that operates in accordance with an external control signal Sc so as to generate and output a predetermined constant voltage Vo1. The external control signal Sc is input to a constant voltage generator circuit part 101. A capacitor 102 for noise control is provided to the output end of the constant voltage circuit 100.
FIG. 2 is a circuit diagram showing a conventional constant voltage power supply circuit that uses a capacitor C10 for noise removal and charges the capacitor C10 quickly (for instance, Japanese Examined Utility Model Application Publication No. 4-26899).
However, the constant voltage circuit 100 shown in FIG. 1 has a problem in that the current used therein is extremely small so that it takes time before the capacitor 102 for noise control is charged to a predetermined voltage, thus requiring time before the output voltage of the constant voltage circuit 100 reaches the predetermined constant voltage Vo1. On the other hand, the constant voltage power supply circuit shown in FIG. 2 requires a circuit that detects that the capacitor C10 for noise removal reaches a predetermined voltage, that is, the voltage Vz of a Zener diode D1.